For example, in accordance with increase of the demand for large-capacity data transmission, the research and development of a digital coherent optical transmission system that enables data transmission more than 100 (Gbps) by a single wavelength light are performed. In the digital coherent optical transmission system, not only an intensity of light but also a phase of the light is used for signal modulation, unlike an intensity modulation system. For example, DP (Dual-Polarization)-QPSK (Quaternary Phase Shift Keying) is mentioned as such a modulation system.
A receiver of the digital coherent optical transmission system detects a signal received from a transmission line by using a local oscillation light, samples the detection result by a clock signal, and then compensate a waveform distortion of the signal caused in the transmission line. In such a receiver, there is known a function that performs a phase synchronization by detecting a phase shift of the sampling from the signal in which the waveform distortion is compensated, and controlling the clock signal on the basis of the phase shift (see e.g. Japanese Laid-open Patent Publication No. 2011-9956).
Moreover, when a nonlinear compensator (NLC) is provided in the receiver, there is known an effect that the waveform distortion of the signal caused by a nonlinear optical effect in the transmission line is compensated and an OSNR (Optical Signal to Noise Ratio) tolerance is improved (See e.g. T. Oyama, et al., “Impact of Pulse Shaping and Transceiver Electrical Bandwidths on Nonlinear Compensated Transmission,” in Optical Fiber Communication Conference/National Fiber Optic Engineers Conference 2013, OSA Technical Digest (online) (Optical Society of America, 2013), paper OTh3C.2.).